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<title>Toolchain technical notes</title>
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<?dbhtml filename="toolchaintechnotes.html"?>
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-<para>This section attempts to explain some of the rationale and technical
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-details behind the overall build method. It's not essential that you understand
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-everything here immediately. Most of it will make sense once you have performed
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-an actual build. Feel free to refer back here at any time.</para>
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-
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-<para>The overall goal of <xref linkend="chapter-temporary-tools"/> is to provide a sane,
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-temporary environment that we can chroot into, and from which we can produce a
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-clean, trouble-free build of the target LFS system in
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-<xref linkend="chapter-building-system"/>. Along the way, we attempt to divorce ourselves
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-from the host system as much as possible, and in so doing build a
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-self-contained and self-hosted toolchain. It should be noted that the
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-build process has been designed to minimize the risks for
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-new readers and provide maximum educational value at the same time. In other
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-words, more advanced techniques could be used to build the system.</para>
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-
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-<important>
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-<para>Before continuing, you really should be aware of the name of your working
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-platform, often also referred to as the <emphasis>target triplet</emphasis>. For
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-many folks the target triplet will probably be
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-<emphasis>i686-pc-linux-gnu</emphasis>. A simple way to determine your target
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-triplet is to run the <command>config.guess</command> script that comes with
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-the source for many packages. Unpack the Binutils sources and run the script:
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-<userinput>./config.guess</userinput> and note the output.</para>
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-
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-<para>You'll also need to be aware of the name of your platform's
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-<emphasis>dynamic linker</emphasis>, often also referred to as the
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-<emphasis>dynamic loader</emphasis>, not to be confused with the standard linker
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-<command>ld</command> that is part of Binutils. The dynamic linker is provided
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-by Glibc and has the job of finding and loading the shared libraries needed by a
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-program, preparing the program to run and then running it. For most folks the
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-name of the dynamic linker will be <filename>ld-linux.so.2</filename>. On
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-platforms that are less prevalent, the name might be
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-<filename>ld.so.1</filename> and newer 64 bit platforms might even have
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-something completely different. You should be able to determine the name
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-of your platform's dynamic linker by looking in the
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-<filename class="directory">/lib</filename> directory on your host system. A
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-sure-fire way is to inspect a random binary from your host system by running:
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-<userinput>readelf -l <name of binary> | grep interpreter</userinput>
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-and noting the output. The authoritative reference covering all platforms is in
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-the <filename>shlib-versions</filename> file in the root of the Glibc source
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-tree.</para>
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-</important>
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-
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-<para>Some key technical points of how the <xref linkend="chapter-temporary-tools"/> build
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-method works:</para>
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-
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-<itemizedlist>
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-<listitem><para>Similar in principle to cross compiling whereby tools installed
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-into the same prefix work in cooperation and thus utilize a little GNU
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-<quote>magic</quote>.</para></listitem>
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-
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-<listitem><para>Careful manipulation of the standard linker's library search
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-path to ensure programs are linked only against libraries we
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-choose.</para></listitem>
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-
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-<listitem><para>Careful manipulation of <command>gcc</command>'s
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-<filename>specs</filename> file to tell the compiler which target dynamic
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-linker will be used.</para></listitem>
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-</itemizedlist>
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-
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-<para>Binutils is installed first because the <command>./configure</command> runs of both GCC and Glibc perform various
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-feature tests on the assembler and linker
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-to determine which software features to enable
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-or disable. This is more important than one might first realize. An incorrectly
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-configured GCC or Glibc can result in a subtly broken toolchain where the impact
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-of such breakage might not show up until near the end of the build of a whole
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-distribution. Thankfully, a test suite failure will usually alert us before too
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-much time is wasted.</para>
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-
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-<para>Binutils installs its assembler and linker into two locations,
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-<filename class="directory">/tools/bin</filename> and
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-<filename class="directory">/tools/$TARGET_TRIPLET/bin</filename>. In reality,
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-the tools in one location are hard linked to the other. An important facet of
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-the linker is its library search order. Detailed information can be obtained
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-from <command>ld</command> by passing it the <parameter>--verbose</parameter>
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-flag. For example: <command>ld --verbose | grep SEARCH</command> will
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-show you the current search paths and their order. You can see what files are
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-actually linked by <command>ld</command> by compiling a dummy program and
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-passing the <parameter>--verbose</parameter> switch to the linker. For example:
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-<userinput>gcc dummy.c -Wl,--verbose 2>&1 | grep succeeded</userinput>
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-will show you all the files successfully opened during the linking.</para>
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-
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-<para>The next package installed is GCC and during its run of
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-<command>./configure</command> you'll see, for example:</para>
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-
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-<blockquote><screen><computeroutput>checking what assembler to use... /tools/i686-pc-linux-gnu/bin/as
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-checking what linker to use... /tools/i686-pc-linux-gnu/bin/ld</computeroutput></screen></blockquote>
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-
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-<para>This is important for the reasons mentioned above. It also demonstrates
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-that GCC's configure script does not search the PATH directories to find which
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-tools to use. However, during the actual operation of <command>gcc</command>
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-itself, the same search paths are not necessarily used. You can find out which
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-standard linker <command>gcc</command> will use by running:
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-<userinput>gcc -print-prog-name=ld</userinput>.
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-Detailed information can be obtained from <command>gcc</command> by passing
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-it the <parameter>-v</parameter> flag while compiling a dummy program. For
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-example: <userinput>gcc -v dummy.c</userinput> will show you detailed
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-information about the preprocessor, compilation and assembly stages, including
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-<command>gcc</command>'s include search paths and their order.</para>
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-
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-<para>The next package installed is Glibc. The most important considerations for
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-building Glibc are the compiler, binary tools and kernel headers. The compiler
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-is generally no problem as Glibc will always use the <command>gcc</command>
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-found in a PATH directory. The binary tools and kernel headers can be a little
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-more troublesome. Therefore we take no risks and use the available configure
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-switches to enforce the correct selections. After the run of
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-<command>./configure</command> you can check the contents of the
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-<filename>config.make</filename> file in the
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-<filename class="directory">glibc-build</filename> directory for all the
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-important details. You'll note some interesting items like the use of
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-<parameter>CC="gcc -B/tools/bin/"</parameter> to control which binary tools are
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-used, and also the use of the <parameter>-nostdinc</parameter> and
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-<parameter>-isystem</parameter> flags to control the compiler's include search
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-path. These items help to highlight an important aspect of the Glibc package:
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-it is very self-sufficient in terms of its build machinery and generally does
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-not rely on toolchain defaults.</para>
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-
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-<para>After the Glibc installation, we make some adjustments to ensure that
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-searching and linking take place only within our <filename class="directory">/tools</filename>
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-prefix. We install an adjusted <command>ld</command>, which has a hard-wired
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-search path limited to <filename class="directory">/tools/lib</filename>. Then
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-we amend <command>gcc</command>'s specs file to point to our new dynamic
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-linker in <filename class="directory">/tools/lib</filename>. This last step is
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-<emphasis>vital</emphasis> to the whole process. As mentioned above, a
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-hard-wired path to a dynamic linker is embedded into every ELF shared
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-executable. You can inspect this by running:
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-<userinput>readelf -l <name of binary> | grep interpreter</userinput>.
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-By amending <command>gcc</command>'s specs file, we are ensuring that every
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-program compiled from here through the end of this chapter will use our new
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-dynamic linker in <filename class="directory">/tools/lib</filename>.</para>
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-
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-<para>The need to use the new dynamic linker is also the reason why we apply the
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-Specs patch for the second pass of GCC. Failure to do so will result in the GCC
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-programs themselves having the name of the dynamic linker from the host system's
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-<filename class="directory">/lib</filename> directory embedded into them, which
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-would defeat our goal of getting away from the host.</para>
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-
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-<para>During the second pass of Binutils, we are able to utilize the
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-<parameter>--with-lib-path</parameter> configure switch to control
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-<command>ld</command>'s library search path. From this point onwards, the
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-core toolchain is self-contained and self-hosted. The remainder of the
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-<xref linkend="chapter-temporary-tools"/> packages all build against the new Glibc in
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-<filename class="directory">/tools</filename> and all is well.</para>
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-
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-<para>Upon entering the chroot environment in <xref linkend="chapter-building-system"/>, the
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-first major package we install is Glibc, due to its self-sufficient nature that
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-we mentioned above. Once this Glibc is installed into
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-<filename class="directory">/usr</filename>, we perform a quick changeover of
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-the toolchain defaults, then proceed for real in building the rest of the
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-target LFS system.</para>
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-
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-<sect2>
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-<title>Notes on static linking</title>
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-
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-<para>Most programs have to perform, beside their specific task, many rather
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-common and sometimes trivial operations. These include allocating memory,
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-searching directories, reading and writing files, string handling, pattern
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-matching, arithmetic and many other tasks. Instead of obliging each program to
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-reinvent the wheel, the GNU system provides all these basic functions in
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-ready-made libraries. The major library on any Linux system is
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-<emphasis>Glibc</emphasis>.</para>
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-
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-<para>There are two primary ways of linking the functions from a library to a
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-program that uses them: statically or dynamically. When a program is linked
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-statically, the code of the used functions is included in the executable,
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-resulting in a rather bulky program. When a program is dynamically linked, what
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-is included is a reference to the dynamic linker, the name of the library, and
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-the name of the function, resulting in a much smaller executable. (A third way
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-is to use the programming interface of the dynamic linker. See the
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-<emphasis>dlopen</emphasis> man page for more information.)</para>
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-
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-<para>Dynamic linking is the default on Linux and has three major advantages
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-over static linking. First, you need only one copy of the executable library
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-code on your hard disk, instead of having many copies of the same code included
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-into a whole bunch of programs -- thus saving disk space. Second, when several
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-programs use the same library function at the same time, only one copy of the
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-function's code is required in core -- thus saving memory space. Third, when a
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-library function gets a bug fixed or is otherwise improved, you only need to
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-recompile this one library, instead of having to recompile all the programs that
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-make use of the improved function.</para>
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-
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-<para>If dynamic linking has several advantages, why then do we statically link
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-the first two packages in this chapter? The reasons are threefold: historical,
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-educational, and technical. Historical, because earlier versions of LFS
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-statically linked every program in this chapter. Educational, because knowing
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-the difference is useful. Technical, because we gain an element of independence
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-from the host in doing so, meaning that those programs can be used
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-independently of the host system. However, it's worth noting that an overall
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-successful LFS build can still be achieved when the first two packages are
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-built dynamically.</para>
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-
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-</sect2>
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+<para>See testing</para>
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</sect1>
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Manuel Canales Esparcia